Certain communication systems have a local station and a remote station drawing its operating power from the local station. That is, the remote station has no power source other than the local station. It is often convenient if only a single pair of conductors connects the stations. For example, there may already be existing only a single pair of conductors, and adding another pair will be costly. Thus, for a system comprising a local station providing power to and communicating with a remote station with only a single pair of conductors connecting them, both data and power must be carried on that same pair of conductors.
Carrying both power and data on the same pair of conductors creates a more complex problem, but one which has previously been solved in a number of different ways. In these earlier systems, the communication is uniplex, i.e., is in only one direction, from the local to the remote station, or vice versa. For example, U.S. Pat. Nos. 3,659,277 (Brown) and U.S. Pat. No. 5,635,896 (Tinsley et al) both disclose a local receiver which provides power to and receives data from a remote transmitter.
In certain of these situations where a local station provides power for both itself and a remote station, it would be more useful if duplex (two way) communication between the stations was available. Best of all would be a system with full duplex operation, where simultaneous communication in both directions is possible, rather than half duplex, where communication in only one direction at any given instant is possible.
One situation in particular where a single pair of conductors carrying both power and full duplex communication has utility is in detecting flame within boilers and other combustion devices. The most common technologies now in use include a discharge tube to detect UV radiation emanating from the flame. Such discharge tubes are located in close proximity to and in line of sight of the combustion chamber and require a pair of conductors for connection to the flame signal processor. There are alternative solid state UV detectors which often require a local preamplifier to process the small signals generated by variations in the UV radiation emitted by the flame. Since two wires are already available, it is much easier in the retrofit situation to use only the existing wiring. Further, if new wiring is installed to increase the number of wires between the flame signal processor and the flame detector, there is additional cost and increased possibility of miswiring. Communication between the flame detector and the processor allows testing of the detector. Power must be supplied to the detector to operate the sensor in some cases, and to operate a preamplifier.